1. Technical Field of the Invention
The present invention relates generally to a waveform shaping circuit for shaping a waveform of an input analog signal into a desired form. More particularly, the invention is directed to an improved waveform shaping circuit which may be employed in an electronic control system for an automotive engine to waveform-shape an alternating analog signal, such as a crank angle signal indicative of engine speed and detected by an electromagnetic pickup, into binary-coded signals exactly corresponding to an alternating cycle of the detected analog signal.
2. Background Art
An electronic control system for an internal combustion engine of an automotive vehicle is conventionally designed to monitor engine speed so as to control ignition timing and fuel injection operations. An electromagnetic pickup is commonly used to measure the engine speed. The electromagnetic pickup detects an angular position of a crank shaft and provides an analog signal which alternates sychronously with rotation of the crank shaft. The alternating analog signal is then converted by a waveform shaping circuit into a binary-coded signal to allow the electronic control system to determine the engine speed.
In order to control engine operating conditions sensor signals indicative of such conditions as coolant temperature and battery voltage, and control signals such as an electric load signal, an idling signal, and an engine starter signal, are utilized in the electronic control system. These signals will, however, produce unwanted electrical disturbances or noise which may interfere with the activity of the waveform shaping circuit, leading to error in determining engine speed. Consequently a proposed waveform shaping circuit provides ramp current to an input alternating analog signal in synchronism with high and low level periods of a binary-coded signal to provide a hysteresis component. This promotes a difference in level between the input analog signal and a reference signal greater than the noise levels caused by the above discussed external signals thus eliminates the influence of the noise.
However, this proposed prior art waveform shaping circuit further encounters another drawback as discussed below. The magnitude and phase of noise superimposed on an output signal of the electromagnetic pickup vary according to changes in engine speed. Therefore, the prior art circuit arrangement wherein constant hysteresis is given to the input analog signal, is unable to eliminate the influence of the noise over a wide range of the engine speeds. Additionally, noise also varies according to engine operating conditions and environmental conditions such as engine coolant temperature, battery voltage, ON/OFF states of electric loads and fuel cut operation. The prior art circuit arrangement does not consider the influence caused by such various noise factors at all.